humanmade/Custom-Metaboxes-and-Fields-for-WordPress — comment on commit e565509f65a3bdd5c39d430fd80215e4d2d01160
File:  line 0
By GaryJones:

Escaping should be done, as late as possible, when it is being used a) on output, and b) in an attribute that takes a URL as a value - not on saving. As such, it should be up to the theme that displays the image or other object element to do the escaping - no text_url type would be specifically needed.